Coke is a solid carbon fuel and carbon source used to melt and reduce iron ore in the production of steel. In one process, known as the “Thompson Coking Process,” coke is produced by batch feeding pulverized coal to an oven that is sealed and heated to very high temperatures for 24 to 48 hours under closely-controlled atmospheric conditions. Coking ovens have been used for many years to convert coal into metallurgical coke. During the coking process, finely crushed coal is heated under controlled temperature conditions to devolatilize the coal and form a fused mass of coke having a predetermined porosity and strength. Because the production of coke is a batch process, multiple coke ovens are operated simultaneously.
The melting and fusion process undergone by the coal particles during the heating process is an important part of coking. The degree of melting and degree of assimilation of the coal particles into the molten mass determine the characteristics of the coke produced. In order to produce the strongest coke from a particular coal or coal blend, there is an optimum ratio of reactive to inert entities in the coal. The porosity and strength of the coke are important for the ore refining process and are determined by the coal source and/or method of coking.
Coal particles or a blend of coal particles are charged into hot ovens, and the coal is heated in the ovens in order to remove volatile matter (“VM”) from the resulting coke. The coking process is highly dependent on the oven design, the type of coal, and the conversion temperature used. Typically, ovens are adjusted during the coking process so that each charge of coal is coked out in approximately the same amount of time. Once the coal is “coked out” or fully coked, the coke is removed from the oven and quenched with water to cool it below its ignition temperature. Alternatively, the coke is dry quenched with an inert gas. The quenching operation must also be carefully controlled so that the coke does not absorb too much moisture. Once it is quenched, the coke is screened and loaded into rail cars or trucks for shipment.
Because coal is fed into hot ovens, much of the coal feeding process is automated. In slot-type or vertical ovens, the coal is typically charged through slots or openings in the top of the ovens. Such ovens tend to be tall and narrow. Horizontal non-recovery or heat recovery type coking ovens are also used to produce coke. In the non-recovery or heat recovery type coking ovens, conveyors are used to convey the coal particles horizontally into the ovens to provide an elongate bed of coal.
As the source of coal suitable for forming metallurgical coal (“coking coal”) has decreased, attempts have been made to blend weak or lower quality coals (“non-coking coal”) with coking coals to provide a suitable coal charge for the ovens. One way to combine non-coking and coking coals is to use compacted or stamp-charged coal. The coal may be compacted before or after it is in the oven. In some embodiments, a mixture of non-coking and coking coals is compacted to greater than 50 pounds per cubic foot in order to use non-coking coal in the coke making process. As the percentage of non-coking coal in the coal mixture is increased, higher levels of coal compaction are required (e.g., up to about 65 to 75 pounds per cubic foot). Commercially, coal is typically compacted to about 1.15 to 1.2 specific gravity (sg) or about 70-75 pounds per cubic foot.
Horizontal Heat Recovery (“HHR”) ovens have a unique environmental advantage over chemical byproduct ovens based upon the relative operating atmospheric pressure conditions inside HHR ovens. HHR ovens operate under negative pressure, whereas chemical byproduct ovens operate at a slightly positive atmospheric pressure. Both oven types are typically constructed of refractory bricks and other materials in which creating a substantially airtight environment can be a challenge because small cracks can form in these structures during day-to-day operation. Chemical byproduct ovens are kept at a positive pressure to avoid oxidizing recoverable products and overheating the ovens. Conversely, HHR ovens are kept at a negative pressure, drawing in air from outside the oven to oxidize the coal's VM and to release the heat of combustion within the oven. It is important to minimize the loss of volatile gases to the environment, so the combination of positive atmospheric conditions and small openings or cracks in chemical byproduct ovens allow raw coke oven gas (“COG”) and hazardous pollutants to leak into the atmosphere. Conversely, the negative atmospheric conditions and small openings or cracks in the HHR ovens or locations elsewhere in the coke plant simply allow additional air to be drawn into the oven or other locations in the coke plant so that the negative atmospheric conditions resist the loss of COG to the atmosphere.
HHR ovens have traditionally been unable to turn down their operation (e.g., their coke production) significantly below their designed capacity without potentially damaging the ovens. This restraint is linked to temperature limitations in the ovens. More specifically, traditional HHR ovens are at least partially made of silica brick. When a silica oven is built, burnable spacers are placed between the bricks in the oven crown to allow for brick expansion. Once the oven is heated, the spacers burn away and the bricks expand into adjacency. Once HHR silica brick ovens are heated, they are never allowed to drop below the silica brick thermally-volume-stable temperature, the temperature above which silica is generally volume-stable (i.e., does not expand or contract). If the bricks drop below this temperature, the bricks start to contract. Since the spacers have burned out, a traditional crown can contract up to several inches upon cooling. This is potentially enough movement for the crown bricks to start to shift and potentially collapse. Therefore, enough heat must be maintained in the ovens to keep the bricks above the thermally-volume-stable temperature. This is the reason why it has been stated that a HHR oven can never be turned off. Because the ovens cannot be significantly turned down, during periods of low steel and coke demand, coke production must be sustained. Further, it can be difficult to perform maintenance on heated HHR ovens. Other portions of the coke oven system can suffer from similar thermal and/or structural limitations. For example, the crown of a sole flue running under the oven floor can collapse or otherwise suffer from heaving of the oven floor, ground settling, thermal or structural cycling, or other fatigue. These stresses can cause bricks in the sole flue to shift and drop out.